1. Field of the Invention
The present invention relates to a centrifugal fan, and more particularly, to a centrifugal fan having a casing and an impeller.
2. Description of the Related Art
A centrifugal fan (centrifugal blower) is a fan for blowing air in a radial direction by rotating an impeller including a plurality of blades (also referred to as wings, impeller). One of this kind of fans is a centrifugal multi-blade fan which includes a casing having a suction opening and a discharge opening and accommodating therein an impeller having a plurality of blades around a rotary shaft of a motor. The centrifugal multi-blade fan suctions air from the suction opening, allows the air flow through the blades from the center of the impeller, and discharges the air outward in the radial direction of the impeller by a centrifugal action from the rotation of the impeller. The air discharged from the outside of the outer circumference of the impeller passes through the casing while increasing the pressure of the air, and the high-pressure air is discharged from the discharge opening.
These centrifugal multi-blade fans are widely used for cooling, ventilation, and air-conditioning in home appliances, office equipment, and industrial equipment, and in blowers for vehicles and the like. The blowing performance and noise of such centrifugal multi-blade fan are largely affected by a blade shape of an impeller and a shape of a casing.
The following patent application publications disclose improvement in blade shapes of fans, for example.
JP-A-2006-207595 discloses a technique for suppressing air from flowing back from a gap formed by an upper case having a bell mouth and a shroud in a centrifugal blower. In other words, the centrifugal blower has the bell mouth formed in the vicinity of an air suction opening of the upper case which accommodates a fan, and the bell mouse has a substantially semi-circular arc in a cross section such that the gap between the upper case and an upper end portion of the shroud narrows.
JP-A-H9-242696 discloses a centrifugal blower for reducing noise of the entire centrifugal blower while suppressing a separation phenomenon between blades and air flowing through blades. That is, the centrifugal blower has a bell mouth ring which is formed in the vicinity of an outer side of a shroud in a radial direction and has a deflection wall surface. The deflection wall surface is configured to deflect air which is discharged outward in the radial direction from a centrifugal multi-blade fan and flowing inwardly toward a rotary shaft, toward the motor side such that the air flows along an inner wall of a casing on the suction opening side. In this manner, it is possible to suppress air from flowing back from a gap between a shroud and the casing to the suction opening. Therefore, it is possible to reduce noise generated due to the interference between air suctioned from the suction opening and the back-flow air, and disturbance of a flow generated when the air flows back in the gap.
JP-A-2004-360670 discloses a centrifugal multi-blade blower capable of preventing disturbance of a flow in the vicinity of a suction opening. That is, the multi-blade blower is a blower for suctioning an air from a direction of a rotation axis, and discharging the fluid in a direction intersecting with the rotation axis, and includes an impeller and a bell mouth. The impeller rotates around the rotation axis. The bell mouth has a suction opening formed to face the impeller, and a recess which is recessed toward the impeller to form a negative-pressure space around the suction opening, and guides a suctioned air to the impeller.
JP-A-2004-190535 discloses a centrifugal blower which suppresses an air flow from being disturbed at a bell mouth portion. That is, the centrifugal blower has an outer wall surface of a scroll casing in which a suction-side outer wall surface connected to the bell mouth portion is formed in a flat shape with no difference in level. In this way, it is possible to suppress disturbance such as a vortex from occurring in suctioned air from flowing toward a suction opening. Therefore, it is possible to suppress an air flow from being disturbed at the bell mouth portion, and thus it is possible to prevent a new vortex loss, noise, and the like from being induced.
As apparatuses have been reduced in sizes and thicknesses, have increased in assembly densities, and have been reduced in power consumption, it has been strongly required from the market to improve static pressures and efficiency for fan motors for those apparatuses. As for fans, it is also important to reduce noise. Particularly, related-art centrifugal fans tend to cause high discrete frequency noise (narrowband noise) and high wideband noise, so that large noise is caused when the centrifugal fans are installed in apparatuses.
Here, the discrete frequency noise is noise based on a blade passing frequency, and is also called as NZ noise. The discrete frequency noise is noise having a characteristic peak at a specific frequency of a narrow frequency band. This frequency can be expressed by the equation: fnz=n (rotational frequency)×z (number of blades). Since not only the primary component but also the secondary and higher components occur, the discrete frequency noise becomes a big problem even in actual hearing. In other words, when those centrifugal fans are installed in apparatuses, there is a risk that noise might occur as clear sound. Also, since a turbulent flow is a dominant factor of wideband noise, and determines a total noise level, it is also required to reduce the wideband noise.
Further, in addition to implementation of the above requirements, it is also required to improve the productivity of fans.
Those techniques disclosed in the above publications are designed for a scroll casing-type fan, and it is also desired to improve a fan having an open-type casing.